I want to have an extension, named .content, to store all files in it that are linked to the filename. The extension ".content" should be recognized by the OS as a file extension rather than a regular map. The files in the extension should be only reached by opening the extension by the option "Show Package Contents".
I hope someone can offer me a solution to my problem.

This is Off topic here this is a question about writing an application. SO better on StackOverflow. However as a start look at Apple's bundle doc
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MarkApr 29 '13 at 18:00

Mark - If the implied question is how to develop an app, then yes, this is off-topic, however there is lots of room to answer how things work and perhaps let the OP edit an existing app to see if they can tweak the launch services database to suit their needs. It's more of a "not detailed enough" question and I wouldn't object to it being closed for that reason since more detail is needed to really know what the OP wants.
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bmike♦Apr 29 '13 at 18:18

OS X ships with a set of extensions that map to specific applications (and most of these map to Apple provided apps) so you could petition Apple to make the change, but that would be unlikely.

The other mechanism to inject a specific .extension to Application mapping into the Launch Services database is to install an Application that tells the system to map certain file types and extensions to that specific Application.

So, to keep things on-topic here let's just direct you to Stack Overflow to ask how to make your own Application. If you're not looking to develop an application, you could also just modify the app you want to open your custom file extension by changing it's interal info.plist file: